FIG. 21 is a circuit diagram of a semiconductor device. A delay circuit 401 delays a signal PX1 to output a signal MZ1. A delay circuit 402 delays a signal PX2 to output a signal MZ2. A delay circuit 403 delays a signal PX3 to output a signal MZ3. A gate of a p-channel field effect transistor 421 receives a signal LCUTX via an inverter 423. A gate of an n-channel field effect transistor 422 receives the signal LCUTX via the inverter 423 and an inverter 424.
To reduce power consumption of the semiconductor device, the transistors 421 and 422 for leakage cut are inserted between source terminals of the delay circuits 401 to 403 and a power supply line, and in a standby period, the transistors 421 and 422 are turned off, which enables a reduction in a leakage current in the standby period.
FIG. 22 is a graph illustrating a refresh period vs. temperature. A horizontal axis represents temperature [° C.] and a vertical axis represents time. To reduce a standby current at room temperatures or lower, a temperature sensor is mounted on a DRAM, and a refresh period TR is changed depending on temperature. The refresh period TR of the DRAM is changed in two stages according to whether the temperature is higher than 60° C. or the temperature is 60° C. or lower. A data retention time tREF of memory cells of the DRAM generally has a temperature characteristic that it becomes longer as the temperature gets lower and its rate of increase is saturated at a certain temperature or lower. In accordance with such a temperature characteristic of the data retention time tREF, when the temperature becomes a determination temperature set in the temperature sensor or lower, the self-refresh period TR is increased, which enables a reduction in a refresh current at room temperatures or lower.
In the semiconductor device in FIG. 21, the leakage current in the standby period can be reduced, but an AC (alternating) current increases due to the control for changing ON/OFF of the transistors 421 and 422 for leakage cut depending on whether a current state in the standby period is a refresh operation period or the refresh non-operation period (hereinafter, referred to as leakage cut control).
FIG. 23 is a chart illustrating temperature characteristics of standby currents. Here, the leakage current and the AC current when the control in FIG. 21 and the control in FIG. 22 are both performed are illustrated. A horizontal axis represents temperature [° C.] and a vertical axis represents current. A leakage current 1104 represents the leakage current in the standby period when the leakage cut control is not performed and thus the transistors 421 and 422 are constantly kept on (hereinafter, referred to as an off leakage current). A current 1103 is a current equal to the sum of a leakage current 1101 and an AC current 1102 and represents a current in the standby period when the leakage cut control is performed. The leakage current 1101 is a leakage current in the standby period when the leakage cut control is performed. The AC current 1102 is an AC current of the transistors 421 and 422 for leakage cut control. At temperatures of 60° C. or lower, since the refresh period TR is long, the AC current 1102 is small, and at temperatures higher than 60° C., since the refresh period TR is short, the AC current 1102 becomes large.
A case is given where, when the temperature sensor determines that the temperature is 60° C. or lower, the refresh period TR is twice as long and the AC current 1102 is reduced to ½ compared with those when the temperature is higher than 60° C. Generally, the off leakage current 1104 changes exponentially with temperature. At a high temperature of about 85° C., since the AC current 1102 ascribable to the leakage cut control is smaller than the off leakage current 1104, performing the leakage cut control enables the total standby current 1103 to be smaller than the off leakage current 1104 by a current difference 1105. However, at room temperature of about 40° C. or lower, compared with the case when the temperature is higher than 60° C., the off leakage current 1104 reduces at a rate of a digit according to an exponential function, while the AC current 1102 reduces only to about ½, and therefore, the AC current 1102 becomes larger than the off leakage current 1104 by a current difference 1106, resulting in an increase in the total standby current 1103. Therefore, there is a problem that it is not possible to reduce the standby current 1103 at room temperatures or lower.
Further, a patent document 1 below describes a semiconductor memory circuit including: an inner circuit to which an operating voltage can be selectively supplied or stopped via a switch and which includes a memory array; and an input circuit receiving a predetermined control signal to control the supply and stop of the operating voltage by the switch.
Further, a patent document 2 below descries a semiconductor memory device in which a power supply voltage of memory cells is made lower than a power supply voltage of a peripheral circuit.
Further, a patent document 3 below describes a semiconductor integrated circuit device which includes a MOSFET and a source potential control circuit controlling a source potential of the MOSFET according to an operation mode of the MOSFET, the source potential control circuit changing the source potential that it controls, based on temperature.    Patent document 1: Japanese Laid-open Patent Publication No. 2003-68079    Patent document 2: Japanese Laid-open Patent Publication No. 04-319598    Patent document 3: Japanese Laid-open Patent Publication No. 2006-12968